Untreated waste gases generated in the manufacture of trinitrotoluene contain tetranitromethane and various nitrogen oxides as pollutants. The prior art procedures are to scrub these waste gases with water and concentrated sulfuric acid, removing the nitrogen oxides but not the tetranitromethane. The treated waste gases, which still contain tetranitromethane, are then discharged into the atmosphere. However, it has been determined that tetranitromethane is an undesirable pollutant which should be removed from the waste gases before they are vented to the atmosphere.
Further, although processes for converting tetranitromethane into trinitromethane are known in the prior art, trinitromethane is still expensive and the supply is limited and erratic. For instance, one prior art process uses aqueous potassium hydroxide to prepare trinitromethane (nitroform) from tetranitromethane [see Schmidt, Ber. 52,400[1919]]. The usefulness of this process is reduced, however, by the simultaneous conversion of a part of the tetranitromethane to potassium carbonate with a corresponding reduction in the yield of the desired trinitromethane product in accordance with the following equations: EQU C(NO.sub.2).sub.4 + 20H.sup.-.fwdarw. C(NO.sub.2).sub.3 .sup.- + NO.sub.3 .sup.-+ H.sub.2 O [1] EQU c(no.sub.2).sub.4 + 6 oh.sup.- .fwdarw. co.sub.3.sup..sup.-2 + 4no.sub.2 .sup.- + 3h.sub.2 o [ii]
the relative yields of the two sets of products formed in these competing reactions depend on the concentrations of tetranitromethane and of potassium hydroxide. The prior art states that unless very high concentrations (over 7 normal) of potassium hydroxide are used, the yield of trinitromethane will be less than 80 percent of the tetranitromethane present in the solution.
This prior art potassium hydroxide process is unsuitable for scrubbing waste gases. First, the aqueous potassium hydroxide solution is not efficient in removing tetranitromethane from waste gases because of the low solubility of the tetranitromethane in the solution. Second, high concentrations of hydroxide ion cause the product alkali metal trinitromethide to decompose. This decomposition may not be very noticeable in laboratory syntheses which take only a short time; however, in a scrubbing process where the product will be exposed to the high hydroxide concentrations (over about 3 N) for hours or even days at a time, most or all of the product will decompose. Yet, low concentrations will also provide poor efficiency and yield. For instance, 0.4 N to 0.5 N aqueous sodium hydroxide will remove only about half of the tetranitromethane from waste gases and convert only 75 percent of that to sodium trinitromethide.
In summary, an aqueous alkali metal hydroxide solution alone is inefficient in removing tetranitromethane and converting it into trinitromethane or its salts because (1) tetranitromethane is not very soluble in the aqueous alkali metal hydroxide solutions and (2) some of the tetranitromethane is converted into carbonate ions by a simultaneous side reaction. Therefore, it would be desirable to provide a process which would efficiently and economically remove tetranitromethane from waste gases and convert it into trinitromethane or its salts.